Image intensifier camera tube having an improved electron bombardment induced conductivity camera tube target comprising a chromium buffer layer

ABSTRACT

An image intensifier camera tube of the type having an image intensifier section for accelerating photoemitted electrons to average energies greater than 1000 electron volts and for focusing the accelerated electrons to one face of a charge storage target which is scanned on its other face by an electron beam. The improvement comprises a buffer layer of chromium on the one face of the target on which the accelerated electrons impinge.

United States Patent Henry et a1.

IMAGE INTENSIFIER CAMERA TUBE HAVING AN IMPROVED ELECTRON BOMBARDMENTINDUCED CONDUCTIVITY CAMERA TUBE TARGET COMPRISING A CHROMIUM BUFFERLAYER Inventors: William Nelson Henry; William Meigs Kramer, both ofLancaster, Pa.

Assignee: RCA Corporation, New York, NY.

Filed: Feb. 11, 1972 Appl. No.: 225,520

U.S.Cl 3l5/l0,3l5/11,315/l2, 313/65 T Int. Cl. I-I0lj 31/26 Field ofSearch 315/10, 11, 12; 313/65 R, 66, 107, 65 T; 250/213 References CitedUNITED STATES PATENTS 6/1966 MacKenzie et a1. 313/65 T [451 Sept. 25,1973 3,363,126 1/1968 Schaefer 313/65 T 3,584,251 6/1971 Banks et a1313/65 T 2,171,213 8/1969 Janes 313/66 3,366,816 1/1968 Saum 313/65 T3,433,994 3/1969 Gibson, Jr 313/10 3,440,476 4/1969 Crowell et a1.315/10 3,628,080 12/1971 Lindequist 315/10 X Primary Examiner-Carl D.Quarforth Assistant ExaminerP. A. Nelson Attorney-Glenn I-I. Bruestle,Donald S. Cohen and Sanford J. Asman [57] ABSTRACT 5 Claims, 3 DrawingFigures PATENTEDSEPZSIQH Fia.

38 32 4O 42 fie. 3

FIG. 1 I

IMAGE INTENSIFIER CAMERA TUBE HAVING AN IMPROVED ELECTRON BOMBARDMENTINDUCED CONDUCIIVITY CAMERA TUBE TARGET COMPRISING A CHROMIUM BUFFERLAYER BACKGROUND OF THE INVENTION The invention relates to imageintensifier camera tube targets.

Image intensifier camera tubes generally comprise an image intensifiersection and a camera tube section. In the image intensifier section, aphotocathode emits electrons in response to a light image. The electronsare accelerated and focused as an electron image on one face, the inputface, of a charge storage target. A large number of secondary chargecarriers are generated in the bulk of the target by the acceleratedelectrons. These secondary carriers diffuse to the other face, theoutput face, of the target and are stored there as an intensified chargepattern. The intensified charge pattern is read from the target with anelectron beam by the camera tube section, which is essentially a vidiconcamera tube.

In one type of image intensifier camera tube, the photoemitted imageelectrons are electrostatically focused and accelerated to averageenergies of several thousand electron volts before striking the inputface of a monocrystalline silicon vidicon target wafer having an arrayof charge storage diodes on its output face. Such an image intensifiercamera tube is described, for instance, in U. S. Pat. No. 3,440,476issued to M. H. Crowell et al. on 22 Apr. 1969. An electrostaticallyfocused image tube which may be adapted for use as the image intensifiersection by substituting the camera tube target input face for thesurface of the phosphor ordinarily on the output faceplate of the imagetube is described, for instance, in U. S. Pat. No. 3,280,356 issued toR. G. Stoudenheimer et al. on 18 Oct. 1966.

It is generally desirable that the image intensifier section be operatedwith the anode cone and the input face of the target at several thousandvolts positive with respect to the photocathode. At voltages lower thanabout a thousand volts, the accelerated electrons become poorly focusedat the target, resulting in loss of resolution. However, when electronswith average en'- ergies of more than about 1,000 electron volts strikethe semiconductor wafer, there are so many secondary carriers generatedthat the charge storage capability of the target becomes quicklysaturated, even with a relatively low light level input to thephotocathode. The result is a loss of gray-scale in the output signal ofthe tube.

One present approach to preventing the loss of grayscale is to apply abuffer layer of some material, such as aluminum metal, to the input faceof the target to absorb some of the energy of the electrons before theyimpinge on the silicon, so that less secondary carriers are generated inthe target. However, the addition of a buffer layer to the target hasheretofore created local non-uniformities in the operatingcharacteristics of the target. These non-uniformities result in amottling, or smudging, in the displayed image signal from the target.

SUMMARY OF THE INVENTION The novel image intensifier camera tubecomprises a target having a buffer layer of chromium metal on its inputface.

Whereas the prior aluminum buffer layers resulted in non-uniformitiessuch as mottling in the signal from the target, the chromium metalbuffer layer of the novel tube performs the desired buffeiing functionof absorbing the desired portion of the energy of the acceleratedelectrons, while having no noticeable effect on the operation uniformityof the target.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cut-away, sectional viewof an image intensifier camera tube in accordance with one embodiment ofthe invention.

FIG. 2 is an exaggerated, sectional view ofa fragment of the target ofthe tube of FIG.

FIG. 3 is an exaggerated sectional view of a fragment of a target inaccordance with another embodiment of the invention.

PREFERRED EMBODIMENT OF THE INVENTION EXAMPLE I One embodiment of thenovel image intensifier camera tube is shown in FIG. 1 of the drawings.The tube 10 has an image intensifier section 12 and a camera tubesection 14. The image intensifier section 12 is es- 'sentially aconventional image inverter diode image tube without its outputfaceplate and phosphor screen. It has a fiber optic bundle faceplate 16having a flat exterior face 18 and a concave interior face 20. Aphotocathode 22 is deposited on the interior face 20. In a centralportion of the image intensifier section 12 is an anode cone 24 foraccelerating electrons emitted from the photocathode 22. At the oppositeend of the image intensifier section 12 is a silicon diode arrayphotoconductive charge storage target 26. The target 26 is positionedwith an input face 28 in the electron image output plane of the imageintensifier section 12, where the accelerated electrons are in focus.

The camera tube section 14 is a vidicon which has, instead of a glassfaceplate, the image intensifier section 12 sealed to the end of itsenvelope. The camera tube section 14 includes inside the base end anelectron gun, not shown, for generating a beam of electrons 30 whichscans the target 26 on its output face 32.

FIG. 2 shows an exaggerated sectional view of a fragment of the target26. The target 26 comprises a monocrystalline wafer 34 of silicon about8 micrometers thick. On the input face 28 of the target 26, there is anevaporated buffer layer 36 of chromium metal about 60 nanometers thick.On the output face 32 of the target 26, there is formed an array ofdiscrete PN junctions 38 in the wafer 34 separated by a web ofinsulating material 40 on the surface of the wafer between the junctions38. Degenerately doped silicon contact pads 42 contact each junction 38and overlap to some extent the insulating material 40. The contact pads42 are scanned by the electron beams 28. The tube 10 is operated withvoltages typical for the two sections 12, 14. For example, the anodecone 24 and the buffer layer 36 are preferably about 3,000 voltspositive with respect to the photocathode 22 and about 10 volts positivewith respect to the cathode of the electron gun.

In operation of the tube 10, light passing through the fiber opticfaceplate l6 and to the photocathode 22 results in the emission ofphotoelectrons 43 in a pattern corresponding to the image input. Thephotoelectrons 43 are accelerated through the anode cone 24 and to thetarget input face 28. The trajectories of the electrons 43 cross overone another at the input opening of the anode cone and result in aninverted, focused electron image at the input face 28 of the target 26.Upon striking the chromium buffer layer 36 on the input face 28 of thetarget 26, the photoelectrons 43 give up a portion of their energy tothe buffer layer 36 before entering the silicon bulk of the target wafer34, where they result in the generation of a large number of secondarycharge carriers. The positive secondary charge carriers are swept to thenearest of the PN junctions 38 on the output face 32 of the target 26,which are reverse-biased, where they result in a stored charge patterncorresponding to the input electron image. This stored charge pattern isread from the target by scanning the output face 32 with the electronbeam 30 in a conventional maner, generally as described in the patent toM. H. Crowell et al. cited above.

EXAMPLE ll In another embodiment of the novel image intensifier cameratube 10, the input face 28 of the target 26 is provided with a firstlayer 44 of chromium about 20 nanometers thick and a second layer 46 ofaluminum about 100 nanometers thick on top of the chromium layer 44 asshown in FIG. 3. We have found that it is desirable to use a layer 44 ofchromium at least 7.5 nanometers thick to obtain uniformity, although itmay be somewhat thicker. For example, the chromium layer 44 may bebetween 7.5 and about 20 nanometers thick and the aluminum layer 46between about 80 and about 110 nanometers thick.

GENERAL CONSIDERATIONS Electron bombardment induced conductivity targetsfor vidicons, generally, are operated by establishing a potentialdifference between the input and output faces. This may be done byfixing the input face voltage at a target voltage of between about 5-50volts positive with respect to ground. The output surface is scannedwith an electron beam from a cathode at ground potential to charge theoutput surface to a ground equilibrium potential. Positive carriers, orholes, are generated locally in the target by elemental portions of theinput electron image and diffuse to the output face, where theydischarge to some extent the local equilibrium potential. Every l/30thsecond, the local area is again scanned by the beam to reestablishequilibrium potential there. The extent of recharging necessary toreestablish equilibrium potential determines the strength of the signalportion that is generated at the local area. Thus, where no inputelectrons are incident, the local area remains substantially atequilibrium potential and no signal results. Where the input image ismost intense, in the highlight areas of the image, the local areas ofthe target are completely discharged within the 1/30 second, to resultin maximum signal. At levels less than the highlight level, there arevarious lesser signal strengths resulting, these representing variousshades of gray. When portions of the input image other than highlightportions result in the generation of so many holes that there iscomplete discharge of the local output areas within the l/30th of asecond, these portions of the image will result in the maximum signalstrength, just as well as do the highlights. Thus, areas which should bea shade of gray will appear white, so that there is a loss ofgray-scale. A lowering of the target voltage can compensate to someextent for such a loss of gray-scale, but it also results in a decreasedtotal signal strength and a lowered signal-to-noise ratio. A betterapproach is to decrease the average energy of the input electron image.This can be done by lowering the accelerating voltages of the imageintensifier section. However, the design of electrostatic imageintensifier sections most suitable for an image intensifier camera tubeuse is such that when the average energy to which the image electronsare accelerated falls below about l,000 electron volts, the electronimage becomes poorly focused at the input face of the target. Yet, whenthe electrons are accelerated to average energies of a thousand or moreelectron volts in the intensifier section, even though there is a lowlight level input to the photocathode, the average energy of theelectrons is still sufficient to result in a substantial loss ofgray-scale in the tube.

When a buffer layer is provided on the target, the average energy of theelectrons becomes relatively noncritical, since the image intensifiersection may now be operated at its optimum voltage. The particularchoice of material for a buffer layer, as well as its thickness,determines how much energy is lost by an electron passing through it.Aluminum seems at first to be a good material, since it is alreadywidely used as an electron permeable coating on the phosphor screens ofimage intensifier tubes and kinescopes. It has been found, however, thataluminum interacts chemically with a number of semiconductor target bulkmaterials, especially with silicon. The interaction changes theelectrical characteristics of the input face portion of the target in anon-uniform manner, so that the output singal is degraded by shading, ormottling.

A buffer layer of chromium does not result in a degraded signal. It maybe applied for example, by evaporation. Any change of electricalcharacteristics of the target due to the chromium is uniform, so that noshading is present in the signal. Chromium is not as reactive as isaluminum, and does not chemically interact noticeably with thesemiconductor materials which are most suitable for electron bombardmentinduced conductivity targets. Moreover, the chromium is especiallyadvantageous for silicon targets, since it is particularly compatiblechemically with it, and since its coefficient of thermal expansionclosely matches that of silicon.

It may be desirable to add one or more layers of other materials on thelayer of chromium to form a composite buffer layer, as in example llabove. The chromium layer will prevent reaction of the other layers withthe target wafer material. For composite buffer layers, the chromiumlayer should be at least 7.5 nanometers thick in order to provide auniform coating. The thickness of the other layers is chosen to providethe desired energy loss for the image electrons. For the example llabove, it was found that the chromium layer should be between 7.5 andabout 20 nanometers thick, and the aluminum layer on the chromium shouldbe between about and about nanometers thick. When chromium alone is usedfor the buffer layer, as in example 1 above, the chromium should bebetween about 52 and about 65 nanometers thick when the average energyof the image electrons are between 2,000 and 4,000 electron volts. Foraverage electron energies of 3,000 electron volts and a silicon diodearray target, the preferred thickness of the chromium buffer layer isabout 60 nanometers. Other electrostatically focused image intensifiersections achieve optimum performance when the image electrons areaccelerated to average energies of about 12,000 electron volts or more.For such energies the buffer layer is made much thicker.

The target may be any type of charge storage target which exhibitselectron bombardment induced conductivity and is otherwise compatiblewith a chromium buffer layer. Most materials presently contemplated forcharge storage targets fall into this category. The storage mechanismmay be an array of heterojunctions, a single junction, or simply aphotoconductive layer without junction.

The chromium layer may be applied by methods other than evaporation. Theparticular method used should be one which will not result in damage tothe target material and which provides a sufficiently uniform thicknessof chromium metal.

We claim: 1. An image intensifier camera tube of the type having:

an evacuated envelope having a transparent faceplate; a photocathode onthe inside surface of said faceplate;

means for accelerating electrons emitted from said photocathode toaverage energies greater than one thousand electron volts and forsubstantially focusing said accelerated electrons in a plane in saidenvelope;

a thin target disposed with one face substantially in said plane andhaving at its other face means for storing charge carriers generated inthe bulk of said target, and

means for scanning said other face of said target with an electron beam,wherein the improvement comprises a continuous layer of chromium on saidone face of said target.

2. The camera tube defined in claim 1 wherein said target comprises asubstantially monocrystalline wafer of'semiconductor.

3. The camera tube defined in claim 2 wherein said layer has a thicknessof between about 52 and about 65 nanometers.

4. The camera tube defined in claim 2 wherein said layer is a compositelayer comprising a first layer of chromium having a thickness of betweenabout 7.5 and about 20 nanometers on said semiconductor, and a secondlayer of aluminum having a thickness of between about and about 1 10nanometers on said chromium layer.

5. The camera tube defined in claim 1 wherein said target comprises asubstantially monocrystalline wafer of silicon having on said one face alayer of chromium and having at its other face an array of chargestorage diodes, said layer of chromium having a thickness of betweenabout 52 and about 65 nanometers.

2. The camera tube defined in claim 1 wherein said target comprises asubstantially monocrystalline wafer of semiconductor.
 3. The camera tubedefined in claim 2 wherein said layer has a thickness of between about52 and about 65 nanometers.
 4. The camera tube defined in claim 2wherein said layer is a composite layer comprising a first layer ofchromium having a thickness of between about 7.5 and about 20 nanometerson said semiconductor, and a second layer of aluminum having a thicknessof between about 80 and about 110 nanometers on said chromium layer. 5.The camera tube defined in claim 1 wherein said target comprises asubstantially monocrystalline wafer of silicon having on said one face alayer of chromium and having at its other face an array of chargestorage diodes, said layer of chromium having a thickness of betweenabout 52 and about 65 nanometers.